A Novel Switchable Microwave Filter Constructed with Inter-Coupled Split-Ring Resonators and Photosensitive Cadmium Sulfide

A novel construction of optically controlled microwave filter is proposed and studied. Inter-coupled split-ring resonators (SRR) were directly implemented on a microstrip transmission line to build a bandpass filter with a very large fractional bandwidth. Highly photosensitive cadmium sulfide (CdS) thin film was utilized to provide the capability of switching to the structure. A low sheet resistance of 300 ohms per square was achieved for CdS thin film under illuminated condition. When the photosensitive CdS thin film is deposited on the microstrip line, the filtering phenomenon induced by the inter-coupled SRR can be controlled optically. It has been demonstrated in both simulation and experimental measurement that the transmission coefficient is switched from 19 dB to 1.5 dB at 5 GHz while the passband region is centered at 14 GHz. Simulation results indicate that the insertion loss can be improved when the SRR cell number or the ring spacing is decreased.